1. Field of the Invention
The present invention concerns a registration aid for medical images, and in particular, a registration aid that is applied to (worn by) a patient from whom medical images are obtained with two different imaging modalities at separated points in time, that allows the contains of the respective images to be brought into registration with each other.
2. Description of the Prior Art
Up to the beginning of the 1970s, medical diagnostic imaging was the domain of classical x-ray technology with projection images. Ultrasound imaging as well as the imaging of nuclear medicine were still at the beginning of their development. X-ray computed tomography (CT) arrived in the seventies, magnetic resonance tomography (MRT) in the eighties. CT and MRT provide cross-section images that show different properties of the examined tissue. CT shows the properties of the spatially-dependent x-ray radiation attenuation of the examined tissue, MRT shows properties that relate to the nuclear spin of the atoms in the tissue, primary hydrogen atoms. Nuclear medical imaging has also been further developed in the direction of the cross-section image representation in parallel with the development of CT and MRT. Imaging by measurement of the intensity of gamma radiation or of photons that emanate from tissue locations in which material specifically emitting this type of radiation was enriched by means of an applied radio pharmaceutical is an example (SPECT=single photon emission computerized tomography). Another nuclear-medical method is PET (position emission tomography), in which a positron-emitting material is administered in a comparable manner to tissue locations, and the emitted positrons combine with an electron in the immediate proximity of their emission locations and thus generate gamma radiation. Imaging with ultrasound has also made significant advances.
The aforementioned non-nuclear medical methods all address different tissue properties or the same tissue properties but with different sensitivity, specificity and the type of the representation. Depending on the problem at hand, a specific method is advantageously applied; for specific problems the application of multiple methods can also decisively increase the diagnostic information.
Nuclear medicine images (SPECT, PET) show intensity distributions in the tissue cross-section mapped in them. These intensity distributions (depending on the composition of the radio pharmaceutical) can represent healthy or normally-functioning tissue (such as, for example, the heart muscle) based on its metabolism or can indicate pathological tissue (such as carcinoma metastases in the liver or in bones) based on a pathological metabolism. Clearly-shown structures are absent or only poorly indicated in non-nuclear medical images, such that a metabolically-active tissue shown by an intensity distribution cannot be classified or can only be insufficiently classified in the morphology of the body cross-section. For specific diagnostic questions, it would be of particular advantage to know the precise location of an intensity distribution within an organ, that could be visualized as if the nuclear-medical slice image and the non-nuclear medical slice image were brought into registration. The purpose of the image registration is to transform the locations of the same tissue or organ points of the body or tissue cross-section, shown in both imaging types or image modalities, such that they have the same spatial coordinates in a common spatial coordinate system or that, in this sense, such locations in the coordinate system of the one modality are even transformed into the coordinate system of the other modality. Such a transformation generally is not achieved solely with a rotation and/or displacement because it is to be assumed that the border of the body cross-section in the image, like the shown organs, is shown differently in the image acquisition processes with both modalities, even when the image section is at the same level in the patient positioning (for example by displacement of inner organs). The registration here is a difficult process.
The process of merging of both images for “integrated display” of both image contents follows the process of the registration. This process of merging is designated as image fusion. This occurs by superimposition of both (registered) images, and for better differentiation of the different content, for example, the nuclear-medical image is applied in color and thus is overlaid on the black-and-white CT image. Since here images of different modalities are registered and merged, this can be considered as a multimodal registration and fusion, but because two modalities are involved, it is more accurate to use the term bimodal registration and fusion.
This process and its fundamental importance are explained using FIG. 1 and FIG. 2, wherein for the selected example the ideal case is assumed that, in the transition from the generation of the Ct image according to FIG. 1a to the acquisition of the PET image according to FIG. 1b, the patient cross-section has experienced no change, for example due to organ displacement or even due to different positioning. FIG. 1a shows a cross-section 1 at the body trunk in a highly schematized manner, with the representation of the liver 2, the aorta 3, the spinal column 4 and both kidneys 5 and 6. The structures that are not further designated represent lung tissue (the large, dark checkered area), the ribs and parts of the digestive system located in the cross-section. The structure 8 represents the pancreas, in which a radiologist should diagnose a cancer source due to its shape and its structure. The expansion of the this source should now be established with a PET exposure and, most notably, the question should be answered of whether the source is limited to the pancreas or is drawing into the stomach artery 7 which (according to FIG. 1a) abuts the aorta 2 on one side and the pancreas 8 at another.
The PET image for the same patient cross-section is shown in FIG. 1b with the radioactive region 9 in an area that is to be associated with the pancreas, and both radioactive regions 10 and 11 that are obviously located in the border areas of the kidneys and which may occur here due to the elimination process for the radio-pharmaceutical proceeding through the kidneys.
If the PET image according to FIG. 1b is successfully superimposed (“fusion”) on the CT image according to FIG. 1a by ensuring the same imaging scales and the same spatial image orientation (“registration”) in a spatially-accurate manner, the fused image is obtained as shown in FIG. 1c. In the example (lacking color representation in the drawings) in the FIG. 1c, the tonal value “white” was associated with the region 9′ that corresponds to the radioactive region 9 in FIG. 1b; the kidney portions characterized as radioactive in FIG. 1b, namely the regions 10 and 11 in FIG. 1c, were also adopted with similar characterization as the regions 10′ and 11′ (also in order to indicate the spatially-correct superimposition).
In this example, the result of the image fusion is not be just that the cancer diagnosis according to the CT image was confirmed by the PET examination, but rather also that the extent of the source is shown with the important visualization that the source transgresses the organ boundaries of the pancreas 8 and encroaches into the stomach artery 7.
The example using FIG. 1a through FIG. 1c applies in a practically identical manner for the imaging type SPECT in place of that of PET. The same would apply if MRT took the place of CT. In principle, other imaging types could also be combined with one another, for example CT and ultrasound. In the case of a combination of different imaging types, multimodal image registration and fusion are referred to in contrast to monomodal (where, for example, images of the same imaging type that were acquired at different points in time are fused with one another in order, for example, to monitor the course of a treatment).
A comprehensive overview of image registration is found in the article “A Survey of Medical Image Registration” by J. B. A. Maintz and M. A. Viergever in Medical Image Analysis, Oxford University Press 1998.
Another example for monomodal image registration and fusion is now discussed. The object is to reliably track a disease or healing progress with by means of CT for soft tissue disease in the skull. In a reference exposure showing the initial situation, the image window (or, more accurately, the signal level window) is selected such that exclusively or predominantly the cranial bone is shown. With successive exposures after specified time periods or after procedures, the cranial bone is initially shown with the same image window and is brought into registration (by image rotation or displacement) with the reference image selected earlier. The image window is then adjusted such that critical soft tissue changes can be detected in the best possible manner. For example, a subtraction of both images to produce a different image that shows the changes that have occurred in the meantime can then be helpful for the comparison.
The above discussion did not take into account the problem that a tilting of the patient's head with regard to his longitudinal axis thereof in successive acquisitions is not always preventable even with diligent efforts. “Landmarks” or “landmark providers” appearing in the image that remain fixed on the skull of the patient over the duration of the monitoring examinations (for example by screws in the bones) provided a solution. This ensures that only identical skull cross-sections (namely characterized by the same occurrence of the landmarks in the images) are compared.
Registration is also difficult in a monomodal case when a body cross-section must be examined in successive image that has no fixed frame such as the cranial bone for the skull. CT images acquired at different times, even of an identical cross-section of the body trunk, are different when (for example as mentioned above in the body trunk example for multimodal registration), the shown organs and the border of the body cross-section appear different in the images due to the displacement of inner organs in the images. A rigid reference system inherent to the body is not present, on which reference system landmark providers could be applied that could be found again in the image. The registration thus must be based on the shown tissue or organ structures, a process that becomes more complex the farther apart in time the different examinations of the patient are from one another.
The registration is more difficult in the multimodal (more precisely stated, bimodal) case, for example of CT and PET, as was also already recognizable in the example discussed using FIG. 1a through 1c. 
Under the assumption that, in the example according to FIG. 1, organ displacements would have occurred in the transition from the examination according to the image in FIG. 1a with CT to the nuclear-medical examination according to the image in FIG. 1b, instead of the PET image shown in FIG. 1b one such as in FIG. 1d could be prepared. The active regions 9, 10 and 11 shown in FIG. 1b are found again in FIG. 1d as the regions 9″, 10″ and 11″. The body cross-section 1 according to FIG. 1a, characterized in FIG. 1b as body cross-section 1′, has changed in terms of its shape in connection with the organ displacement, as indicated in FIG. 1d as body cross-section 1″. For clarification, parts of the contours of the radioactive regions 9, 10 and 11 shown in FIG. 1b are plotted in equivalence in FIG. 1d as 9*, 10* and 11*.
It can now be attempted to stretch, compress, rotate and displace the image in FIG. 1d in specific image ranges by means of image processing until the kidney contours 10″ and 11″ indicated by the activities come into congruence with the fully-formed contours of the kidneys 5 and 6 in FIG. 1a. However, the question then always exists as to whether, due to this transformation, the topology of the image in FIG. 1d entirely or approximately coincides with that which was displayed in the image in FIG. 1a if it were to be “inherently visible” or, expressed otherwise, whether it coincides with that which was introduced by FIG. 1b into the image of FIG. 1a as area 9 and then appears in FIG. 1c as region 9′. It is still true that the advantageous possibility to effect an adaptation of the body cross-section 1″ in FIG. 1d to the body cross-section 1 in the image of FIG. 1a is not available, because even the body cross-section 1″ in the PET image according to FIG. 1d (and that characterized in the image of FIG. 1b and here as 1′) is not mapped or is less sufficiently mapped.
An algorithm that can be considered as applying a rubber blanket onto the image in FIG. 1d traces the radioactive area 9″ and the kidney contours indicated in the active radiation regions 10″ and 11″ onto the rubber blanket, if applicable after clarification of the kidney contours by an experienced doctor. Such an algorithm would have to rotate this rubber blanket (for example by spatially-dependent dragging) so as to rotate or to shift it as a whole, or only for local image parts (and decreasingly at their boundary regions) until the kidney contours shown (and, if applicable, clarified by manipulation) in FIG. 1d with the regions 10″ and 11″ coincide with those shown in FIG. 1a. Given the development of such an algorithm, it would be necessary to incorporate prior information into it that displace it in terms of position to take into account possible preferential directions in which, for example, the pancreas displaces when one or both kidneys 5 and 6 are shifted in the direction of the inside of the body by displacement of the patient.
An image fusion of CT images and PET images is today is viewed as so important for diagnostics but their correct registration is viewed as so difficult that the mathematical process of a complex image transformation is circumvented by a physical solution. This is to combine CT and PET in one apparatus, such only a displacement of the patient positioning table is needed to transport the patients from one into the other acquisition mode; a repositioning of the patient thus is not done. The acquisitions of the different types are executed in one examination step, one immediately after the other, such that an inherent movement of the organs (for example due to intestinal peristalsis) can also be assumed as not present or as only slight. It is assumed that the cross-section images of both modalities CT and PET incorporated in one apparatus are of practically identical topology.
Combined CT/PET systems are already commercially available; a serially-produced CT/SPECT apparatus was introduced in the middle of 2004 (Medical Solutions including electromedica, November 2004, page 16). The previously employed and continuing observations for a CT/PET apparatus equally apply for a CT/SPECT apparatus.
Such a CT/PET apparatus, however, does not solve the registration problem when primary PET images for the monitoring examinations are indexed for a patient after an operation and/or in the course of a different treatment and when, to reduce or avoid the radiation dose to be applied to the patient and for reasons of additional costs, it is desired to omit the respective CT acquisitions that would otherwise be generated simultaneously. A similar problem exists when a patient for whom an examination with a CT apparatus provides inducement for a further examination with PET is remitted to a clinic or hospital that also operates a CT/PET machine. It can wholly be assumed that the still-new dual-modality systems are not installed or are only installed to a limited extent in radiological institutes. Radiology and nuclear medicine are different profession directions, and normally only nuclear medicine institutes have authorization for the exposure with open radioactive substances such as, for example, radio-pharmaceuticals. The question exists as to whether and which compromises should be made for the application of one of the two modalities when both modalities are integrated in one apparatus. It is thus likely that the above-described double modality systems will not replace the use of apparatus that perform one or the other of the two individual modalities of the dual system.
The problem of an optimal image registration and fusion thus remains.